Systems of adatoms on semiconductor surfaces  display competing ground states and exotic  spectral properties typical of two-dimensional correlated electron materials which are dominated by a complex interplay of spin and charge degrees of freedom. We report a derivation of low-energy Hamiltonians for {Sn, Si, C, Pb}/Si(111). Our solution of these models with GW+DMFT unifies the theoretical interpretation of the materials class and allows for an explanation for long standing seeming discrepancies for the most intriguing material Sn:Si(111) [1-3].

[1] P. Hansmann, L. Vaugier, H. Jiang, and S. Biermann, JPCM 25,094005 (2013).
[2] P. Hansmann, T. Ayral, L. Vaugier, P. Werner, and S. Biermann, Phys. Rev. Lett. 110, 166401 (2013).
[3] P. Hansmann, T. Ayral, A. Tejeda, and S. Biermann, Nature Scientific Reports 6, Article number: 19728 (2016).

Real materials typically have involved bandstructures and interaction paramters so that full Hamiltonians are not solvable. However, even if we could diagonalize the full Hamiltonian we would not have identified the essential degrees of freedom that determine the ground state which could be, e.g. a high Tc superconducting one.  To identify such most relevant degrees of freedom we often start from ab initio (e.g. DFT, Hartree Fock, GW) calculations and integrate out states of high energy remaining with a low energy effective Hamiltonian. For basically all transition metal oxides this procedure leads to the question if and how to include oxygen 2p states explicitely. A famous example is given by the models for high Tc cuprate superconductors.  While various studies of the different models exist, a satisfying unification of the different models (i.e. how observables calculated in the various models relate to one another) is still missing.

Selected publications on oxides:
[1] P. Hansmann et al., Phys. Rev. Lett. 103, 016401 (2009)
[2] P. Hansmann et al., Physical Review B 82, 235123 (2010)
[3] N. Parragh et al., Phys. Rev. B 88, 195116 (2013)
[4] P. Hansmann et al., New J. Phys. 16, 033009 (2014)
[5] P. Seth et al. arXiv:1508.07466 (2015)

Thermoelectric devices have become a hot topic in materials science. This is easy to understand since regaining only a fraction of the energy that is wasted as heat (~60%) would contribute tremendously to a solution of future energy problems. Despite development for more than a
century there are some severe problems concerning efficiency and operating temperatures that have not been solved. An alternative route, coined as the thermoelectronic approach, is investigated by experimentalists in our institute. The key quantity to be optimized for  thermoelectronic devices is the work function of the emitter/collector material. We employ density functional methods to simulate various compounds and to understand how the work function can be manipulated in order to support our experimental colleagues in finding the best  possible materials [1].

[1] Zhicheng Zhong and Philipp Hansmann, Phys. Rev. B 93, 235116 (2016)